a question about gasoline and my odometer

[Bertha Venation]

Every time I fill up my gas tank, I reset the trip odometer.

At 100 miles, I am down to about 7/10ths of a tank.

At 200 miles, I have half a tank left.

Why, when I get to 300, am I running on fumes? Wouldn't it stand to reason that at 300 miles I'd have a quarter of a tank?

Note: I seldom get below a quarter tank before filling up, but sometimes I can't get to the gas station right away. Just like everyone else, I guess.

[NYC_SKP]

1. Most fuel gauges suck. My Prius is the worst I've owned.

There are ten digital squares and after a fill up I can go over 100 miles before a change even registers from ten to nine dots.

As I chew through the tank, they disappear increasingly fast.

For all the cars I've owned that aren't accurate, it's always an error of the same sort, the first quarter tank lasts a LOT longer than the last quarter.

....

[onehandle]

2. Fuel gauges are universally inaccurate. Temperatures can affect them easily.

On a hot day, the gauge will show more gas than on a cold day.

[HopeHoops]

3. I get about 500 miles out of a tank. The trip odometer is the only useful instrument.

Unlike the other computer-based things (fuel gauge, overhead computer, etc.) the trip odometers are tied to the same line as the main odometer so they read accurately. The computer-based thingies on our van have gone wacky. I can fill up and it will read empty with "zero miles" remaining. Other times, it will read full and then suddenly drop to empty. I don't even pay attention to them anymore. The solid rule is "RESET THE FUCKING B TRIP!"

[Scuba]

4. I once bought an Audi 100LS. On my first trip I learned that when the guage read 1/4 full....

... that meant I had about 100 feet left before I ran out of gas.

Thanks goodness for the van full of dope-smoking hippies who stopped and picked me up (midnight on the Pa Turnpike), took me to a gas station and then drove me back to my car. Good people.

I was always filling that sucker up.

[Populist_Prole]

5. It's due to the geometry of the float type sending unit in the tank

Most employ a float style similar to the older toilet tank floats: The float at the end of a lever, rather than the linear slide type of float. The transducer ( basically a potentiometer ) is at the fulcrum of the float lever and measures movement.

What happens is that when the tank is full, the float is at the top and the lever arm is at 90 degrees from vertical ( or possibly slightly higher as the float can be submerged in fuel but unable to be bouyant due to it already being at the top of the tank ). This is one reason you'll seem to go a good long ways with the tank still registering full. The other dynamic is that as the fuel level lowers, the float; being located at the end of the lever, will begin imparting a greater relative angle as the level drops: That is, a 2 inch drop from full will have the potentiometer "seeing" less motion than a 2 inch drop from say a quarter of a tank.

One more thing is this. Due to space packaging requirements, their are some oddly shaped fuel tanks out there and a float has no idea how narrow or wide the tank is at any given point.

Edit: I looked for and found something that explained it better than I could, plus there are pictures:

http://www.cracked.com/article_19571_8-gadgets-that-lie-to-you-every-day.html

[Bertha Venation]

7. Dude! Thank you!

I'm not mechanically inclined, but this made sense to me.

And I love Cracked . . .

[Denninmi]

6. If your car is older, maybe sludge in the bottom, too.

I think that can happen. Takes up volume but provides no energy.